As materials for food cans, beverage cans, aerosol cans, and other containers, metal sheets covered on their surfaces with a polyester, polyolefin, or other thermoplastic resin are being made much use of. In this case, the metal sheet used is generally steel sheet or aluminum sheet. Among these, for example, polyester-based resins are generally superior in corrosion resistance, flaw resistance, and printability and can be used for both the inner surface and outer surface of cans in some cases, but when the content is alkaline, the resin is insufficient in durability. Further, when used for meat-based food cans, there is the problem that the meat releasability is poor. To solve this problem, two-sided laminated metal sheets covered with different types of resin at the can inside surface side and the can outer surface side are being used.
In general, as the can outer surface side film, a relatively hard polyester-based resin film is preferably used, while as the can inside surface side film, a lower melting point, excellent meat releasability and alkali durability, relatively soft polyolefin-based resin film is preferably used. Metal sheet laminated with different films on its two surfaces is disclosed in Japanese Patent Publication (A) No. 63-231926, Japanese National Publication (A) No. 2-501644, and Japanese Patent Publication (A) No. 2002-120324. Note that the terms “high melting point film” and “low melting point film” used in the present description do not mean films with melting points of absolute values. A relatively high melting point side film is called a “high melting point film”, while a relatively low melting point side film is called a “low melting point film”.
In general, a laminated metal sheet is produced by the method of superposing a heated metal sheet and resin films and using lamination rolls to apply pressure to bond them (heat lamination method). To make them bond, the temperature of a film surface contacting the metal sheet has to be at least its melting start point Tsm (normally a temperature about 0 to 30° C. lower than the melting point), more preferably at least the melting point MP, but on the other hand if the temperature of a film surface contacting a lamination roll becomes the melting start point or more, the film will stick to the lamination roll making production impossible.
For this reason, the temperature of the metal sheet at the lamination part has to be strictly controlled in relation with the film melting points, but when using a polyester-based resin film as the high melting point film and using a polyolefin-based resin film as the low melting point film, since the melting points MP of the two greatly differ (for example, polyethylene terephthalate (polyester-based) has a melting point of 265° C., while polypropylene (polyolefin-based) has one of 168° C.), there is the problem that if setting the temperature of the metal sheet to match with one of the films, the other film will not bond well.
Therefore, as shown in Japanese Patent Publication (A) No. 63-231926, the method of first laminating the high melting point resin film, then laminating the low melting point resin film in a later step where the temperature of the metal sheet falls, that is, a two-step lamination method, and, as shown in Japan National Publication (A) No. 2-501644, the method of reheating after the lamination step by the press-bonding of the lamination rolls so as to melt-bond the resins have been proposed, but both of these have the problems of swelling capital costs. Further, Japanese Patent Publication (A) No. 2002-120324 proposes a simultaneous lamination method matching the temperature of the metal sheet with the low melting point resin film, but it is believed that a low melting point resin for bonding purposes is required at the metal sheet side of the high melting point resin film, so again the cost rises.
Further, in the prior art disclosed in Japanese Patent Publication (A) No. 63-231926 and Japanese Patent Publication (A) No. 2002-120324, since the low melting point resin film is kept from melt-bonding with the lamination roll by setting the temperature of the metal sheet to match with the low melting point resin film, the crystallization degree will not be lowered to an extent enabling the laminated low melting point resin film to be sufficiently worked. For this reason, when bending a two-sided laminated metal sheet to work it to a food can etc., the phenomenon of the worked part of the low melting point film whitening appears. It looks like the can contains foreign matter. Therefore, users and can makers would be liable to raise complaints.